Sistem Jaringan
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Data
#6
LAN Architecture
 topologies
 transmission
medium
 layout
 medium
access control
LAN Topologies
Bus and Tree
 used
with multipoint medium
 transmission propagates throughout medium
 heard by all stations
 full duplex connection between station and
tap

allows for transmission and reception
 need

to regulate transmission
to avoid collisions and hogging
 terminator
absorbs frames at end of medium
 tree a generalization of bus
 headend connected to branching cables
Frame
Transmission
on Bus LAN
Ring Topology




a closed loop of repeaters joined by point to
point links
receive data on one link & retransmit on
another


links unidirectional
stations attach to repeaters



circulate past all stations
destination recognizes address and copies frame
frame circulates back to source where it is
removed
data in frames
media access control determines when a
station can insert frame
Frame
Transmission
Ring LAN
Star Topology
 each

usually via two point to point links
 either


 or
station connects to central node
central node can broadcast
physical star, logical bus
only one station can transmit at a time
central node can act as frame switch
Choice of Topology
 reliability
 expandability
 performance
 needs



considering in context of:
medium
wiring layout
access control
Bus LAN
Transmission Media (1)
 twisted



pair
early LANs used voice grade cable
didn’t scale for fast LANs
not used in bus LANs now
 baseband


coaxial cable
uses digital signalling
original Ethernet
Bus LAN
Transmission Media (2)



broadband coaxial cable




as in cable TV systems
analog signals at radio frequencies
expensive, hard to install and maintain
no longer used in LANs



expensive taps
better alternatives available
not used in bus LANs
optical fiber
less convenient compared to star topology twisted
pair
 coaxial
baseband still used but not often
in new installations
Ring and Star Usage
 ring


very high speed links over long distances
single link or repeater failure disables
network
 star



uses natural layout of wiring in building
best for short distances
high data rates for small number of devices
Choice of Medium
 constrained
by LAN topology
 capacity
 reliability
 types
of data supported
 environmental scope
Media Available
 Voice
 Cat
grade unshielded twisted pair (UTP)
3 phone, cheap, low data rates
 Shielded
 more
twisted pair / baseband coaxial
expensive, higher data rates
 Broadband
 even
 High
cable
more expensive, higher data rate
performance UTP
 Cat
5+, very high data rates, witched star
topology
 Optical
fibre
 security,
high capacity, small size, high cost
LAN Protocol Architecture
IEEE 802 Layers (1)
 Physical




encoding/decoding of signals
preamble generation/removal
bit transmission/reception
transmission medium and topology
IEEE
802
Layers
(2)
 Logical Link Control
 interface
to higher levels
 flow and error control
 Media
 on
Access Control
transmit assemble data into frame
 on receive disassemble frame
 govern access to transmission medium
 for same LLC, may have several MAC options
LAN Protocols in Context
Logical Link Control
 transmission
of link level PDUs between
stations
 must support multiaccess, shared medium
 but MAC layer handles link access details
 addressing involves specifying source and
destination LLC users


referred to as service access points (SAP)
typically higher level protocol
Media
Access
Control
 where

central
 greater

control, single point of failure
distributed
 more
complex, but more redundant
 how

synchronous
 capacity

dedicated to connection, not optimal
asynchronous
 in
response to demand
Asynchronous Systems
 round
 each
robin
station given turn to transmit data
 reservation
 divide
medium into slots
 good for stream traffic
 contention
 all
stations contend for time
 good for bursty traffic
 simple to implement
 tends to collapse under heavy load
MAC Frame Handling


MAC layer receives data from LLC layer
fields







MAC control
destination MAC address
source MAC address
LLC
CRC
MAC layer detects errors and discards frames
LLC optionally retransmits unsuccessful frames
Bridges
 connects
similar LANs
 identical physical / link layer protocols
 minimal processing
 can map between MAC formats
 reasons for use




reliability
performance
security
geography
Bridge Function
Bridge Design Aspects
 no
modification to frame content or
format
 no encapsulation
 exact bitwise copy of frame
 minimal buffering to meet peak demand
 contains routing and address intelligence
 may connect more than two LANs
 bridging is transparent to stations
Connection of Two LANs
Bridges and
LANs with
Alternative
Routes
Fixed Routing




complex large LANs need alternative routes

for load balancing and fault tolerance




done in configuration
usually least hop route
only changed when topology changes
widely used but limited flexibility
bridge must decide whether to forward frame
bridge must decide LAN to forward frame to
can use fixed routing for each sourcedestination pair of LANs
Spanning Tree
 bridge
automatically develops routing
table
 automatically updates routing table in
response to changes
 three mechanisms:



frame forwarding
address learning
loop resolution
Frame Forwarding

maintain forwarding database for each port


lists station addresses reached through each port
for a frame arriving on port X:




search forwarding database to see if MAC
address is listed for any port except X
if address not found, forward to all ports except X
if address listed for port Y, check port Y for
blocking or forwarding state
if not blocked, transmit frame through port Y
Address Learning
 can
preload forwarding database
 when frame arrives at port X, it has come form
the LAN attached to port X
 use source address to update forwarding
database for port X to include that address
 have a timer on each entry in database
 if timer expires, entry is removed
 each time frame arrives, source address
checked against forwarding database


if present timer is reset and direction recorded
if not present entry is created and timer set
Spanning Tree Algorithm




address learning works for tree layout
in general graph have loops
for any connected graph there is a spanning
tree maintaining connectivity with no closed
loops
IEEE 802.1 Spanning Tree Algorithm finds this



each bridge assigned unique identifier
exchange info between bridges to find spanning
tree
automatically updated whenever topology
changes
Loop of Bridges
Interconnecting LANs - Hubs
 active
central element of star layout
 each station connected to hub by two UTP
lines
 hub acts as a repeater
 limited to about 100 m by UTP properties
 optical fiber may be used out to 500m
 physically star, logically bus
 transmission from a station seen by all others
 if two stations transmit at the same time
have a collision
Two Level Hub Topology
Buses, Hubs and Switches
 bus


all stations share capacity of bus (e.g. 10Mbps)
only one station transmitting at a time
 hub



configuration
uses star wiring to attach stations
transmission from any station received by hub and
retransmitted on all outgoing lines
only one station can transmit at a time
total capacity of LAN is 10 Mbps
 can
improve performance using a layer 2
switch


can switch multiple frames between separate ports
multiplying capacity of LAN
Shared
Medium
Bus and
Hub
Layer 2 Switch Benefits
 no
change to attached devices to
convert bus LAN or hub LAN to switched
LAN

e.g. Ethernet LANs use Ethernet MAC protocol
 have
dedicated capacity equal to
original LAN

assuming switch has sufficient capacity to keep
up with all devices
 scales

easily
additional devices attached to switch by
increasing capacity of layer 2
Types of Layer 2 Switch
 store-and-forward
switch
 accepts
frame on input line, buffers briefly, routes
to destination port
 see delay between sender and receiver
 better integrity
 cut-through
 use
switch
destination address at beginning of frame
 switch begins repeating frame onto output line as
soon as destination address recognized
 highest possible throughput
 risk of propagating bad frames
Layer 2 Switch vs Bridge
 Layer
2 switch can be viewed as full-duplex hub
 incorporates logic to function as multiport
bridge
 differences between switches & bridges:






bridge frame handling done in software
switch performs frame forwarding in hardware
bridge analyzes and forwards one frame at a time
switch can handle multiple frames at a time
bridge uses store-and-forward operation
switch can have cut-through operation
 hence
bridge have suffered commercially
Layer 2 Switch Problems
 broadcast




users share common MAC broadcast address
broadcast frames are delivered to all devices
connected by layer 2 switches and/or bridges
broadcast frames can create big overhead
broadcast storm from malfunctioning devices
 lack

overload
of multiple links
limits performance & reliability
Router Problems
 typically
routers


use subnetworks connected by
limits broadcasts to single subnet
supports multiple paths between subnet
 routers
do all IP-level processing in
software


high-speed LANs and high-performance layer 2
switches pump millions of packets per second
software-based router only able to handle well
under a million packets per second
Layer 3 Switches
 Solution:
layer 3 switches
 implement
packet-forwarding logic of
router in hardware
 two
categories
 packet
by packet
 flow based
Packet by Packet or
Flow Based
 packet


by packet
operates like a traditional router
order of magnitude increase in performance
compared to software-based router
 flow-based



switch
enhances performance by identifying flows of IP
packets with same source and destination
by observing ongoing traffic or using a special flow
label in packet header (IPv6)
a predefined route is used for identified flows
Typical
Large
LAN
Organization
Diagram